1. Field of the Invention
The present invention concerns a method of improving the wear and corrosion resistance of ferrous metal parts guaranteeing highly reducible results regardless of the type of parts treated.
2. Description of the Prior Art
The great variety of surface treatments is explained in part by the multiplicity of situations with which the technician is faced in industrial practice and by the extreme interdependence of the phenomena causing surface deterioration.
What is more, requirements are becoming ever more severe and an ever greater number of parts must resist loads combining several phenomena, for example, in mechanical engineering: friction, wear, corrosion, even impact and fatigue. In these cases, the conventional methods are found wanting.
Apart from this improvement in performance, under the impulsion of industrial demand, the technician is increasingly concerned with the industrialization of these methods in terms of quality, reliability and reproducibility, in other words with the search for "zero defects".
Thermochemical diffusion treatments are of recognized benefit in improving wear resistance of ferrous metal parts, especially those known as nitriding.
A significant improvement in resistance to wet corrosion can be achieved by growing an oxide surface layer on top of the nitrided layer.
There is an abundant literature on these oxidation treatments. It includes the following patents:
FR-A-2 306 268 of 1976 describes an oxidizing salt bath comprising alkali hydroxides with optionally 2% to 20% by weight alkali nitrate. Preferably employed at temperatures from 200.degree. C. to 300.degree. C., this salt bath is essentially intended for controlled cooling of nitrided ferrous metal parts on leaving a cyanate/cyanide nitriding bath and elimination by oxidation of cyanides entrained with the parts.
FR-A-2 463 821 of 1980 states that the same alkali hydroxide bath, containing 2% to 20% by weight alkali nitrate, significantly increases the corrosion resistance of the nitrided parts if they are immersed in the bath at a temperature between 250.degree. C. and 450.degree. C. for a sufficient length of time, between 15 mins. and 50 mins. A study of this patent, and in particular of its examples, which relate to a bath comprising 37.4% by weight sodium hydroxide, 52.6% by weight potassium hydroxide and 10% by weight sodium nitrate, shows improved resistance to salt spray corrosion, reflected in exposure times prior to the appearance of traces of corrosion that are virtually doubled.
U.S. Pat. No. 4,448,611 to Grellet et al., which corresponds to FR-A-2 525 637 of 1982, describes a method of treating ferrous metal parts containing sulfur in an oxidizing salt bath to improve their corrosion resistance. This document teaches immersion of the parts in an oxidizing bath containing alkali hydroxides, alkali nitrates and/or nitrites and optionally alkali carbonates and additionally 0.5% to 15% by weight of a powerful oxidizing agent, in this instance oxygenated salts of alkali metals whose normal oxy-reduction potential relative to the hydrogen reference electrode is -1 volt or less. The oxygenated salts mentioned are bichromates, permanganates, peroxycarbonates, iodates and periodates, the alkali metals being sodium and potassium. The method described in this patent is further characterized in that a gas containing oxygen is blown into the salt bath and in that the percentage by weight of insoluble particles in the bath is kept below 3%. This method can achieve even better performance. It can improve the corrosion resistance of parts, in this instance by a factor of almost 4, and also does not degrade their resistance to wear and fatigue and can even improve their anti-seizing properties for dry friction.
However, it has become apparent that this performance cannot be achieved in practice with the levels of reliability and reproducibility required by industry. In the laboratory, performance variations are relatively hard to detect. They are much more noticeable when treating industrial size quantities, however. They are particularly apparent when large quantities of small parts must be treated "in bulk", or when treating parts whose surface condition is less than perfect: the presence of imperfections such as pressing and stamping burrs, crimping and bending creases and welding defects are potential corrosion starting sites.
Uneven corrosion resistance is totally unacceptable in the case of parts such as jack piston or damper piston rods and windshield wiper and starter motor spindles for automobiles. The long-standing solution is to adjust the baths from time to time as necessary, according to observed variations in behavior. This solution is unsatisfactory given the industrial requirements as previously explained. This has made it necessary to look for new solutions. We have undertaken an in-depth study which has led to the development of a method which can control on an industrial scale the imponderables previously encountered.
Accordingly, an object of the present invention is a method combining thermochemical diffusion and passivation by oxidation enabling significant improvement in wear and corrosion resistance of ferrous metal parts while guaranteeing a high level of reproducibility and therefore minimal dispersion.